Hydrodynamic bearing device

ABSTRACT

A hydrodynamic bearing device is provided for use with a spindle motor. The hydrodynamic bearing device has a sleeve made of a sintered metal that is obtained by sintering a sintering material that is iron, an iron alloy, copper, a copper alloy or a mixture thereof. This sintered metal has independent pores, which do not communicate with each other, by selecting conditions for forming a desired sintered body within a predetermined range. The conditions includes a grain size of powdered metal of a material for the sintered metal, a molding pressure when the molded body is formed, sintering temperature and sintering period in the sintering step.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 11/195,813, filed Aug.3, 2005, now U.S. Pat. No. 7,513,689.

This application claims priority under 35 U.S.C. §119 to Japanese PatentApplication No. 2004-229825, filed Aug. 5, 2004. The entire disclosureof Japanese Patent Application No. 2004-229825 is hereby incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a hydrodynamic bearing deviceutilizing a dynamic pressure and a method for manufacturing the device.

2. Background Information

In recent years, a storage capacity of a recording apparatus or the likeusing a disk or the like has been increasing, and a data transmissionrate thereof has been increasing as well. A high speed and a highprecision of rotation is necessary for a spindle motor that is used forsuch a recording apparatus. Therefore, a hydrodynamic bearing device isused for a rotation shaft portion of the spindle motor. A conventionalhydrodynamic bearing device will be described below with reference toFIGS. 9 and 10.

FIG. 9 is a cross sectional view of a spindle motor including aconventional hydrodynamic bearing device. As shown in FIG. 9, a sleeve101 having a bearing bore 101 a is made of a sintered metal that is madeof sintering powdered metal, such as a copper alloy. The sleeve 101 ismade of a sintered metal mainly to reduce manufacturing costs. If thesleeve 101 is produced by machining a metal bar or the like, a lot ofchips will be generated as waste material. In contrast, a sintered metaldoes not generate such chips. In addition, the time necessary forproducing a sleeve using a sintered metal is a fraction of the timenecessary for producing the same by machining. Accordingly, theproduction using a sintered metal is suitable for low-cost massproduction.

On the outer surface of the sleeve 101, a sleeve cover 114 is provided.The sleeve cover 114 is made of a metal that is not a sintered metal. Ashaft 102 is inserted in the bearing bore 101 a of the sleeve 101 in arotatable manner. A thrust flange 103 is fixed to a lower end portion ofthe shaft 102. The thrust flange 103 is housed in a space enclosed bythe sleeve 101, the sleeve cover 114 and a thrust plate 104. A lowerface of the thrust flange 103 in FIG. 9 is opposed to the thrust plate104. An upper face of the thrust flange 103 is opposed to a lower endface of the sleeve 101.

A rotor hub 105 is fixed to an upper end portion of the shaft 102. Arotor magnet 106 is fixed to an inner surface of the rotor hub 105. Therotor magnet 106 is opposed to a motor stator 107 that is fixed to abase 108. An inner surface of the bearing bore 101 a of the sleeve 101is provided with dynamic pressure generating grooves 109 a and 109 b inthe radial direction that are well known in the art. In addition, a faceof the thrust plate 104 that is opposed to the thrust flange 103 isprovided with dynamic pressure generating grooves 110 a in the thrustdirection that are also well known. Dynamic pressure generating grooves110 b may be formed on at least one of the opposed faces of the thrustflange 103 and the sleeve 101, if necessary. Oil 111, as working fluid,is filled in the space between the shaft 102 and the sleeve 101,including the dynamic pressure generating grooves 109 a, 109 b, 110 aand 110 b, as well as in the space between the thrust flange 103 and thesleeve 101 and the space between the thrust flange 103 and the thrustplate 104.

An operation of the conventional hydrodynamic bearing device will bedescribed with reference to FIG. 9. When the motor stator 107 issupplied with power, a torque is generated by the rotor magnet 106, sothat the rotor hub 105, the shaft 102 and the thrust flange 103 rotateas one body. As a result of this rotation, the dynamic pressuregenerating grooves 109 a, 109 b, 110 a and 110 b respectively give apumping pressure to the oil 111 in the corresponding portions.Accordingly, radial bearings are formed at the area of the dynamicpressure generating grooves 109 a and 109 b for supporting the shaft 102in the radial direction, while thrust bearings are formed at the area ofthe dynamic pressure generating grooves 110 a and 110 b for supportingthe flange 103 in the thrust direction. Thus, the shaft 102 and theflange 103 rotate without contacting the bearing bore 101 a and thethrust plate 104.

Since the sleeve 101 is made of a sintered metal, it has pores at 2-15%of volume (small spaces contained in the sintered metal). The poresinclude those called “tissue pores” existing inside the sintered metaland those called “surface pores” opening on the surface of the sinteredmetal. In a usual sintered metal, the surface pores and the tissue poresare communicated with each other. Although the sleeve 101 made of thesintered metal is impregnated with oil at a pressure lower than anatmospheric pressure in advance, the oil can pass through the sleeve 101by moving in the pores. In the conventional example, the sleeve 101 issurrounded by the sleeve cover 114 so that the oil does not leakexternally by moving through the pores.

According to the structure of the conventional hydrodynamic bearingdevice shown in FIG. 9, it is necessary to insert the sleeve 101 insidethe sleeve cover 114 in the manufacturing process, which increases thenumber of man-hours in production. Since the sleeve 101 and the sleevecover 111 are individual parts, the number of parts increases and thecost is also increased. In addition, if the sleeve 101 is inserted inthe sleeve cover 114 with an inclined position, as shown in FIG. 10 inthe insertion process, the axis of the bearing bore 101 a is not keptperpendicular to the surface of the thrust plate 104. In this state, thegap of the thrust bearing or the radial bearing shown in FIG. 9 becomesuneven so that the shaft 102 cannot be supported in a stable manner. Ifthe unevenness of the gap increases, the shaft 102 contacts the bearingbore 101 a of the sleeve 101 and the bearing is seized up. The sameproblem can occur if the axis of the bearing bore 101 a of the sleeve101 is not precisely perpendicular to the opposed face of the thrustflange 103 fixed to the shaft 102.

When the shaft 102 rotates in the conventional hydrodynamic bearingdescribed above, a hydraulic pressure within the range of 2-5atmospheric pressures is generated by the radial dynamic pressuregenerating grooves 109 a and 109 b. If the oil is driven by thishydraulic pressure to flow in the pores of the sleeve 101, the hydraulicpressure is reduced to 70% of the above-mentioned pressure. As a result,stiffness of the radial bearing is also reduced to approximately 70%.Japanese Unexamined Patent Publication No. 2003-322145 discloses amethod of covering the entire surface of the sleeve 101 with a coatinglayer that is not permeable to oil in order to prevent the oil fromentering the pores of the sleeve 101. This method includes a step offorming the coating layer. Consequently, the method has many steps and ahigh cost.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a hydrodynamic bearingdevice including a sleeve made of sintered metal that can preventworking fluid, such as oil, from leaking externally. A further object ofthe present invention is to prevent stiffness of the radial bearing fromdecreasing. Still yet another object of the present invention is tomaintain an appropriate bearing gap of a thrust bearing or a radialbearing to ensure stable non-contact rotation.

A hydrodynamic bearing device according to the present inventionincludes a sleeve having a bearing bore, a shaft that is inserted in thebearing bore of the sleeve in relatively rotatable manner, a thrustflange that is provided to an end of the shaft, a thrust member that isopposed to the thrust flange, a dynamic pressure generating grooveformed on the inner surface of bearing bore and is, a dynamic pressuregenerating groove formed on at least one of the opposed surfaces of thethrust flange and a thrust member, and working fluid filled in a gapbetween the shaft and the bearing bore, and between the thrust flangethe thrust member. The sleeve is a sintered body that is obtained bysintering a sintering material containing at least one selected from agroup containing iron, an iron alloy, copper and a copper alloy. Poresof the sintered body are independent pores in which neighboring poresare independent from each other, and a diameter of the independent poresis smaller than each of a width and a height of a crest portion of theradial dynamic pressure generating groove.

According to the present invention, since pores in the sleeve made of asintered body are independent pores, working fluid does not enter thesleeve, and therefore the working fluid does not leak through thesleeve. In addition, since a diameter of the independent pores issmaller than each of a width and a height of a crest portion of theradial dynamic pressure generating groove, functions of the radialdynamic pressure generating groove is hardly deteriorated even if anindependent pore exist in the crest portion.

According to another aspect of the present invention, the hydrodynamicbearing device includes a sleeve having a bearing bore, a shaft that isinserted in the bearing bore of the sleeve in relatively rotatablemanner, a thrust flange that is provided to an end of the shaft, athrust member that is opposed to the thrust flange, a radial dynamicpressure generating groove formed on the inner surface of bearing boreso as to work as a radial bearing and is, a thrust dynamic pressuregenerating groove formed on one of the opposed surfaces of the thrustflange and a thrust member so as to work as a thrust bearing, andworking fluid filled in a gap between the shaft and the bearing bore,and between the thrust flange the thrust member. The sleeve is asintered body that is obtained by sintering a sintering materialcontaining at least one selected from a group containing iron, an ironalloy, copper and a copper alloy. At least one of sintered body formingconditions including a molding pressure the sintered body, sinteringtemperature and sintering period that are sintering conditions, and anaverage grain size of metal grains of the sintering material is selectedso that a diameter of the independent pores becomes smaller than each ofa width and a height of a crest portion of the radial dynamic pressuregenerating groove.

According to the present invention, at least one of sintered bodyforming conditions of the sleeve made of a sintered body including, themolding pressure of the sintered body, the sintering temperature, thesintering period and the average grain size of metal grains of thesintering material is selected so that a diameter of the independentpores becomes smaller than each of a width and a height of a crestportion of the radial dynamic pressure generating groove. Thus, pores ofthe sleeve become independent pores. Accordingly, working fluid does notenter the sleeve, and therefore the working fluid does not leak throughthe sleeve. In addition, since a diameter of the independent pores issmaller than each of a width and a height of a crest portion of theradial dynamic pressure generating groove, functions of the radialdynamic pressure generating groove is hardly deteriorated even if anindependent pore exist in the crest portion.

A method according to the present invention is used for manufacturing ahydrodynamic bearing device including a sleeve having a bearing bore, ashaft that is inserted in the bearing bore of the sleeve in relativelyrotatable manner, a thrust flange that is provided to an end of theshaft, a thrust member that is opposed to the thrust flange, a dynamicpressure generating groove formed on the inner surface of bearing boreand is, a dynamic pressure generating groove formed on one of theopposed surfaces of the thrust flange and a thrust member and is, andworking fluid filled in a gap between the shaft and the bearing bore,and between the thrust flange and the thrust member. The sleeve is asintered body that is obtained by sintering a sintering materialcontaining at least one selected from a group containing iron, an ironalloy, copper and a copper alloy. The method includes the step ofselecting at least one of sintered body forming conditions including amolding pressure when the sintered body is molded, sintering temperatureand sintering period that are sintering conditions, and an average grainsize of metal grains of the sintering material so that pores of thesintered body are independent pores in which neighboring pores areindependent from each other, and that a diameter of the independentpores becomes smaller than each of a width and a height of a crestportion of the radial dynamic pressure generating groove.

According to the present invention, in the molding and sintering stepsfor forming the sintered body of the sleeve, at least one of the moldingpressure of the sintered body, sintering temperature, sintering periodand an average grain size of metal grains of the sintering material isselected so that pores of the sintered body are independent pores inwhich neighboring pores are independent from each other, and that adiameter of the independent pores becomes smaller than each of a widthand a height of a crest portion of the radial dynamic pressuregenerating groove. Thus, pores of the sleeve become independent pores.Accordingly, working fluid does not enter the sleeve, and therefore theworking fluid does not leak through the sleeve. In addition, since adiameter of the independent pores is smaller than each of a width and aheight of a crest portion of the radial dynamic pressure generatinggroove, functions of the radial dynamic pressure generating groove ishardly deteriorated even if an independent pore exist in the crestportion.

According to the present invention, all pores on the outer surface ofthe sintered body that constitutes the sleeve are independent pores, soworking fluid does not enter the sleeve. Accordingly, the working fluiddoes not leak externally through the sleeve made of the sintered body.

Since a diameter of the independent pores is smaller than each of awidth and a height of a crest portion of the radial dynamic pressuregenerating groove, the dynamic pressure generating grooves do not dropout largely due to the independent pores, the functions of the dynamicpressure generating groove are not affected largely.

These and other objects, features, aspects and advantages of the presentinvention will become apparent to those skilled in the art from thefollowing detailed description, which, taken in conjunction with theannexed drawings, discloses preferred embodiments of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of thisoriginal disclosure.

FIG. 1 is a cross sectional view of a spindle motor including ahydrodynamic bearing device according to a first embodiment of thepresent invention.

FIG. 2 is a flow chart showing a manufacturing process of a sleeve inthe first embodiment.

FIG. 3( a) is an enlarged view of a surface of a molded body containinglarge pores that communicate with each other.

FIG. 3( b) is an enlarged view of a surface of a sintered metalcontaining small pores that communicate with each other.

FIG. 3( c) is an enlarged view of a surface of the sintered metalcontaining independent pores.

FIG. 4 is a partial enlarged view of a dynamic pressure generatinggroove having independent pores.

FIG. 5 is a cross sectional view of the hydrodynamic bearing accordingto a second embodiment of the present invention.

FIG. 6 is a cross sectional view of a main portion of a molding die formolding the sleeve in the second embodiment of the present invention.

FIG. 7 is a cross sectional view of a main portion of a molding die formolding the sleeve in the first embodiment of the present invention.

FIG. 8 is a cross sectional view showing a machining process of thesleeve using a lathe.

FIG. 9 is a cross sectional view of a spindle motor including aconventional hydrodynamic bearing.

FIG. 10 is a cross sectional view of a main portion of the conventionalhydrodynamic bearing showing an assembled state.

FIG. 11 is a cross sectional view of a spindle motor including ahydrodynamic bearing device according to an additional embodiment of thepresent invention.

FIG. 12 is a cross sectional view of a spindle motor including ahydrodynamic bearing device according to a further additional embodimentof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Selected embodiments of the hydrodynamic bearing device according to thepresent invention will be described with reference to FIGS. 1-8. Notethat the following embodiments are just examples, and the presentinvention is not limited to these embodiments. In other words, it willbe apparent to those skilled in the art from this disclosure that thefollowing descriptions of the embodiments of the present invention areprovided for illustration only and not for the purpose of limiting theinvention as defined by the appended claims and their equivalents. Inaddition, in the following embodiments, overlapping description can beomitted by assigning the same reference numerals to the same elements.

First Embodiment

Hereinafter, the hydrodynamic bearing device according to a firstembodiment of the present invention will be described with reference toFIGS. 1-4. The hydrodynamic bearing device of the present invention isespecially useful as a spindle motor having a low cost and highreliability.

FIG. 1 is a cross sectional view of a spindle motor including ahydrodynamic bearing device according to a first embodiment of thepresent invention. As shown in FIG. 1, a sleeve 1 having a bearing bore1 a is made of a sintered body or sintered metal produced by sinteringpowdered metal containing at least one of iron, an iron alloy, copperand a copper alloy. The sleeve 1 is fixed to a base 8. A shaft 2 is madeof a metal that is not a sintered body, and is inserted in the bearingbore 1 a in a rotatable manner. The shaft 2 has a thrust flange 3 thatis formed integrally therewith. The sleeve 1 includes a stepped endportion 1 b that is coaxial with the bearing bore 1 a. The stepped endportion 1 b has a diameter larger than a diameter of the bearing bore 1a. The thrust flange 3 is housed in a space enclosed by the stepped endportion 1 b of the sleeve 1 and a thrust plate 4 that is a thrustbearing member. A lower surface of the thrust flange 3 is opposed to thethrust plate 4. A rim portion of an upper surface of the thrust flange 3is opposed to the stepped end portion 1 b of the sleeve 1.

A rotor hub 5 is fixed to an upper end portion 2 a of the shaft 2. Arotor magnet 6 of the spindle motor is fixed to the rotor hub 5. A motorstator 7 that is opposed to the rotor magnet 6 is fixed to the base 8.Radial dynamic pressure generating grooves 9 a and 9 b that are wellknown in the art are formed on at least one of an inner surface of thebearing bore 1 a of the sleeve 1 and an outer surface of the shaft 2. Afirst thrust dynamic pressure generating groove 10 a is formed on asurface of the thrust plate 4 that is opposed to the lower surface ofthe thrust flange 3. In addition, second thrust dynamic pressuregenerating grooves 10 b are formed on a surface of the stepped endportion 1 b of the sleeve 1 that is opposed to the upper surface of thethrust flange 3. Alternatively, the first or second thrust dynamicpressure generating groove 10 a or 10 b is formed on an opposed surfaceformed on the thrust flange 3. Oil 11, as working fluid, is filled inthe space between the shaft 2 and the bearing bore 1 a of the sleeve 1,including the radial dynamic pressure generating grooves 9 a and 9 b andthe thrust dynamic pressure generating grooves 10 a and 10 b, as well asin the space between the thrust flange 3 and the sleeve 1 and the spacebetween the thrust flange 3 and the thrust plate 4.

An operation of the hydrodynamic bearing device according to the firstembodiment will be described with reference to FIG. 1. When the motorstator 7 is supplied with power, a rotation magnetic field is generatedat the rotor magnet 6, so that the shaft 2 and the thrust flange 3rotate as one body with the rotor hub 5. The radial dynamic pressuregenerating grooves 9 a and 9 b and the thrust dynamic pressuregenerating grooves 10 a and 10 b generate a pumping pressure that iswell known in the art. The shaft 2 and the thrust flange 3 rotatewithout contacting the sleeve 1 and the thrust plate 4 respectively. Inother words, the radial dynamic pressure generating grooves 9 a and 9 bform a radial bearing for supporting the shaft 2 in the radial directionin the bearing bore 1 a without contact between them, while the thrustdynamic pressure generating grooves 10 a and 10 b form a thrust bearingfor supporting the thrust flange 3 in the thrust direction withoutcontacting the sleeve 1 and the thrust plate 4.

With reference to FIG. 2, a general process for producing the sleeve 1from a material of the sintered metal will be described. In the step S1shown in FIG. 2, powdered metal is prepared. The powdered metal ispowder of iron, an iron alloy, copper, a copper alloy or the like. Thematerial of the sintered metal usually has a grain diameter ofapproximately 0.1 mm, and is solely or a mixture of the iron, the ironalloy, the copper or the copper alloy. A lubricant is prepared in stepS2. The lubricant is applied to the powdered metal so that they aremixed uniformly (step S3). The lubricant works as a parting agent whenthe powdered metal is molded. The mixed material is filled in a moldingdie that is formed to the shape of the sleeve 1, and a press or the likeis used for compressing it at a predetermined molding pressure. Thus, amolded body is made by shaping it in a shape of the sleeve 1 (step S4).In the molded body, after the molding, the powdered metal is linked in astate that is nearly point contact. A molding die of the sleeve 1 can beprovided with patterns of the radial dynamic pressure generating grooves9 a and 9 b that are formed on the inner surface of the bearing bore 1 aof the sleeve 1 and patterns of the thrust dynamic pressure generatinggrooves 10 b that are formed on the stepped end portion 1 b. Thus, thedynamic pressure generating grooves can be formed simultaneously whenthe sleeve 1 is molded in the step S4, resulting in very highefficiency. Under present circumstances, the radial dynamic pressuregenerating grooves 9 a and 9 b and the thrust dynamic pressuregenerating grooves 10 b are formed by a machining process aftersintering the sleeve 1.

Next, the molded body is heated to a predetermined temperature so as tobe sintered, and thus, the sintered body is obtained (step S5). Theheating process widens the contact portion among the powdered metal soas to form surface contacts so that the molded body is compressed andthe volume is decreased. As a result, density of the sintered bodyincreases and intensity is improved. Since contraction due to thesintering process causes reduction of size of the component, it isshaped in a little larger in advance and a machining process isperformed, if necessary, so that the sleeve 1 is completed in a desireddimension (step S6).

The state of the “pores”, which are small spaces existing in thesintered body, is changed by molding pressure and a sintering periodwhen the molded body is heated and sintered. In addition, an averagegrain size of the metal grains of the sintering material also affectsthe state of the pores of the sintered body. When the molding pressureand the sintering temperature are increased and the sintering period iselongated, the size of the pores is reduced so that neighboring poresbecome “independent pores” that do not communicate with each other butare independent. The smaller the grain size of the metal grains becomes,the smaller the pores become so that independent pores are easilyformed. The “molding pressure”, the “sintering temperature”, the“sintering period” and the “average grain size of metal grains” of themolded body are referred to as “sintered body forming conditions”. Astate of the pores on the surface (outer surface) of the sintered bodywill be described with reference to FIGS. 3( a), 3(b) and 3(c).

FIG. 3( a) is a partial enlarged view of a molded body 30 a that isobtained by molding powdered metal 12 a containing grains ofapproximately 0.1 mm in diameter using a mold. The powdered metal 12 ais hardened in the state that neighboring grains contact each other atpoint of contact to the molded body 30 a in a predetermined shape. Inthis state, pores 13 a exist between neighboring grains of the metal 12a, so the neighboring pores 13 a communicate each other.

FIG. 3( b) is a partial enlarged view of a sintered body 30 b that isobtained by sintering the molded body 30 a shown in FIG. 3( a) in thesintering step. In the sintering step, the molded body 30 a is heatedfor a predetermined period at a predetermined temperature. Selecting theheating temperature and the heating period appropriately produces thepores substantially as desired in size. As shown in FIG. 3( b),neighboring grains of the powdered metal 12 b contact each other as thepoints of contact are made flat by the sintering step, and the pores 13b have become smaller as a result. In this state, a density of thesintered body 30 b is 70-90% of a real density of the metal materialconstituting the sintered body 30 b. In the case of a sintered body madeof a metal having a real density of approximately 8 (e.g., iron orcopper) for example, the density becomes a value within the range ofapproximately 5.6-7.2. In this state, neighboring pores stillcommunicate with each other, and the sintered body, having such adensity, is generally used as a sintered bearing impregnated with oil.

FIG. 3( c) shows a surface of a sintered body 30 c that is obtained bysintering the molded body 30 a for a longer period (e.g., for threehours) at a higher temperature (e.g., 800 C degrees) than the case shownin FIG. 3( b). The grains of the powdered metal 12 c are deformed to bea substantially hexagonal section. As a result the pores 13 c becomevery small so that neighboring pores 13 c become independent pores thatdo not communicate with each other. A density of the sintered body 30 cin this state becomes a value above 90% of the real density of the metalmaterial. In the case of a sintered body made of a metal having a realdensity of approximately 8 for example, the density becomes a valueabove 7.3. When the sintering is performed so that a density of thesintered body becomes a value close to the real density of the metalmaterial, almost all of the pores become independent pores. In thesleeve 1 made of such a sintered body, almost all of the pores becomeindependent pores. In general, the higher the heating temperature andthe longer the heating period, the smaller the pores become. Thus, thesintered body becomes the same state as a metal block finally, and adensity thereof becomes nearly equal to the real density of the metalmaterial. There are infinite combinations of selection of the averagegrain size of the powdered metal and setting of the sinteringtemperature and the sintering period for making the pores of thesintered body independent pores. The metal material of the sinteredmetal also affects the state of the pores.

According to the present invention, at least one of the sintered bodyforming conditions is changed in accordance with a metal material to beused while manufacturing prototypes. Thus, the pores become independentpores. Sintered body forming conditions are selected so that a size ofthe independent pores becomes smaller than each of the width (W) anddepth (D) of the crest portion of the radial dynamic pressure generatinggrooves 9 a and 9 b as described in detail with reference to FIG. 4. Ifall pores of the sleeve 1 are independent pores, the oil never passesthrough the sleeve 1 and leaks externally. Therefore, it is notnecessary to cover the outer surface of the sleeve 101 with the sleevecover 114 unlike the conventional structure, as shown in FIG. 9.Therefore, a hydrodynamic bearing having high reliability can berealized with a simple structure.

FIG. 4 is a diagram showing a partial enlarged view of the inner surfaceof the bearing bore 1 a of the sleeve 1. The inner surface is providedwith crest portions 14 a, 14 b and 14 c and dynamic pressure generatinggrooves 18 a, 18 b, 18 c and 18 d. A width of the dynamic pressuregenerating grooves 18 a-18 d and a width W of the crest portions 14 a-14c are both approximately 0.2 mm, and a depth D thereof is approximately0.01 mm. The width W and the depth D shown in FIG. 4 are notproportional to the real size.

The crest portion 14 a at the left end in FIG. 4 shows the example wherea pore 17 a having a larger diameter than the width W exists at thecrest portion 14 a. The pore 17 a makes the dynamic pressure generatinggrooves 18 a and 18 b have a continuous large width (three times thewidth W), so the function of the dynamic pressure generating grooves isinsufficient at the vicinity of this pore 17 a.

The crest portion 14 b at the middle in FIG. 4 shows the example where apore 17 b having a width smaller than the width W of the crest portion14 b and a larger depth exists. In this case, oil enters the pore 17 b,and bearing stiffness is reduced at this portion.

The crest portion 14 c at the right end in FIG. 4 shows the examplewhere a pore 17 c having a width much smaller than the width W and adepth smaller than the depth D exists. In this case the pore 17 c hardlyaffects the dynamic pressure generating grooves 18 c and 18 d. Asdescribed above, if a diameter of the independent pores is smaller thaneach of the width W and the depth D of the crest portion that forms thedynamic pressure generating grooves 18 a-18 d, the function of thedynamic pressure generating grooves 18 a-18 d are substantiallymaintained. The crest portion 14 c at the right end in FIG. 4 representsan example according to the present invention.

As another method for obtaining the independent pores, there is a knownmethod of performing a finishing process on the sintered body after thesintering step. In the finishing process, the sleeve 1 of the sinteredbody after the sintering step is put in another die having asubstantially similar shape to the molding die for producing the moldedbody and having dimensions a little smaller than the same so as to applya pressure to the sleeve 1. Grains of the powdered metal 12 a aredeformed by the pressure and form intimate contacts with each other sothat the pores become smaller. In addition, it is possible to perform acold plastic process such as a coining process in which a pressure isapplied to the surface of the sleeve 1 by using a pressing machine afterthe sintering step or a shaving process in which the surface is pressedand rubbed. These cold plastic processes can squash the pores on thesurface of the sleeve 1 to be independent pores. However, compared withthe method of this embodiment, it has a disadvantage in that a lot ofprocess steps are necessary and cost for manufacturing is high. It isalso possible to apply pressure and heat in the molding die when moldingthe sleeve 1. It is effective to make the grain size of the powderedmetal small or to use a sintering material that is a mixture of two ormore kinds of powdered metal having different grain sizes, so that thediameter of the pores in the molded body before sintering becomes assmall as possible. There is a method of applying a special coating onthe surface of the powdered metal so that the pores become even thoughalthough the cost increases. Furthermore, it is possible to plate theentire surface of the sleeve 1 so as to improve corrosion protection andabrasion resistance and to make auxiliary means for making the pores 13independent pores 14.

As described above, since the sleeve 1 is made of a sintered metal inthe hydrodynamic bearing device according to the first embodiment of thepresent invention, its cost is low and its productivity is high. Thedynamic pressure generating grooves of the sleeve 1 are formed in acomponent rolling step as another step.

Since there are pores communicating with each other in the conventionalsintered metal, oil leaks through the sleeve if the sleeve is made ofconventional sintered metal. In the sintered metal of this embodiment,pores are independent pores, so oil does not pass through the sleeve andtherefore does not leak externally.

The diameter of the independent pores is smaller than each of a widthand a depth of the dynamic pressure generating groove. Therefore, evenif there are pores in the crest portion of the dynamic pressuregenerating groove, they do not affect the function of the dynamicpressure generating groove. Thus, a hydrodynamic bearing device low costand high reliability is realized.

Second Embodiment

A hydrodynamic bearing device according to a second embodiment of thepresent invention will be described with reference to FIGS. 5, 6 and 7.FIG. 5 is a cross sectional view of the hydrodynamic bearing accordingto a second embodiment. As shown in FIG. 5, a sleeve 40 having a bearingbore 40 a is a cylindrical sintered body. The inner surface of thebearing bore 40 a is provided with radial dynamic pressure generatinggrooves 9 a and 9 b that are formed by a cold plastic process method(component rolling method) or the like. The lower end face of the sleeve40 in FIG. 5 is provided with thrust dynamic pressure generating grooves10 b. The sleeve 40 has independent pores in the same way as the sleeve1 in the first embodiment.

The thrust member 44 of the thrust bearing portion is a saucer-likemember made of a sintered metal or a metal material that is not asintered metal. The thrust member 44 has a circular recess 44 a. Adiameter and a depth of the recess 44 a are adapted to house the thrustflange 3 that is fixed to the lower end portion of the shaft 2 with atiny gap. The bottom surface of the recess 44 a is provided with dynamicpressure generating grooves 10 c. When the thrust member 44 is made of asintered metal, it is formed by using a sintered body having independentpores similar to the sleeve 40.

The shaft 2 has the same structure as that in the first embodiment. Theshaft 2 is inserted in the bearing bore 40 a so that the thrust flange 3at the lower end portion is opposed to the thrust dynamic pressuregenerating grooves 10 b, and the thrust member 44 is fixed to the sleeve40. The thrust dynamic pressure generating grooves 10 b and 10 c and thethrust flange 3 constitute the thrust bearing portion.

When the hydrodynamic bearing device of the second embodiment isequipped with the hub 5 and the base 8 or the like similar to thestructure shown in FIG. 1, a spindle motor is constituted.

Since the sleeve 40 of the second embodiment has a simple cylindricalshape, it has a characteristic that dimensional accuracy of each portionafter sintering process is higher than that of the sleeve 1 in the firstembodiment. Hereinafter, the molding step of the sleeve 40 will becompared with that of the sleeve 1, so as to explain the higher whydimensional accuracy of the sleeve 40 than the sleeve 1.

Since the sleeve 40 is a sintered body, it is slightly contracted in thesintering step. A quantity of contraction varies in accordance with anunevenness of a density distribution of the molded body, an unevennessof a temperature distribution when temperature rises in the sinteringstep, a variation of the sintering temperature and other factors. Inorder to improve dimensional accuracy of the sintered body, it isnecessary to improve precision of the molding die and to make thedensity distribution of the molded body as uniform as possible. Thedensity distribution of the molded body has a tendency to be uniform asthe shape of the molded body is simpler.

The step for obtaining the molded body of the sleeve by putting thepowdered metal of the sintering material in a molding die and pressingthe same will be described with reference to FIGS. 6 and 7. FIG. 6 is across sectional view of a main portion of a molding die 50 for moldingthe sleeve 40 in the second embodiment. FIG. 7 is a cross sectional viewof a main portion of a molding die 52 for molding the sleeve 1 in thefirst embodiment.

As shown in FIG. 6, a bar-like inner die 20 is fixed inside thecylindrical outer die 21 in a coaxial manner with the outer die 21. Theinternal diameter of the outer die 21 corresponds to the outer diameterof the sleeve 40 shown in FIG. 5, while the external diameter of theinner die 20 corresponds to the inner diameter of the bearing bore 40 a.An upper die 22 and a lower die 23 are disposed inside the outer die 21.The upper die 22 and the lower die 23 move in the direction of arrows 22a and 23 a, respectively, inside the outer die 21. The dimension in FIG.6 is a thickness of the molded body that is obtained by the molding die50, which corresponds to a length L of the sleeve 40 shown in FIG. 5.The dimension 3L indicates a space between the upper die 22 and thelower die 23 when they are opened.

The sintering material obtained in the step S3 shown in FIG. 2 is addedbetween the upper die 22 and the lower die 23. The upper die 22 and thelower die 25, in the state illustrated in FIG. 6, are separated with aspace of the dimension 3L. When the sintering material is added, eitherone of the upper die 22 and the lower die 23 is removed. Next the upperdie 22 is moved in the direction of the arrow 22 a, and the lower die 23is moved in the direction of the arrow 23 a. In other words, the upperdie 22 and the lower die 23 are moved in their respective direction bythe same distance so as to press the sintering material. When the upperdie 22 and the lower die 23 are moved until a gap distance L is formed,the molded body 19 a is obtained. FIG. 6 shows the state where themolding is completed and the molding dies 22 and 23 have returned totheir positions, respectively. Radial dynamic pressure generatinggrooves 9 a and 9 b and thrust dynamic pressure generating grooves 10 bare formed on the molded body 19 a in a component rolling step that isanother step similar to the sleeve 40 shown in FIG. 5.

In the molding step shown in FIG. 6, the sintering material that isplaced between the upper die 22 and the lower die 23 having thedimension 3L is pressed and molded into the ring-like molded body 19 a.In other words, since it is molded into the ring-like shape that is asimple shape, the obtained molded body 19 a has a very uniform densitydistribution with little partial variation of density. In addition, whenplural molded bodies 19 a are produced, a variation of density amongthem is relatively small.

A difference between the right angle and an angle defined by the axis ofa hole 19 b of the molded body 19 a and the bottom face 19 c(hereinafter, a difference between the right angle and an angle isreferred to as perpendicularity) is determined by perpendicularity ofthe axis of the inner die 20 with respect to a press face 23 a of thelower die 23, a gap size between the lower die 23 and the inner die 20,and a gap size between the lower die 23 and the outer die 21. Since eachof the inner die 20, the outer die 21, the upper die 22 and the lowerdie 23 has a simple shape, the above-mentioned gaps can be set toappropriate values so that a desired accuracy can be obtained.Therefore, the perpendicularity of the axis of the hole 19 b withrespect to the bottom face 19 c can be set to an appropriate value, too.Thus, a molded body 19 a having a desired high precision can beobtained.

In the above mentioned embodiment, both the upper die 22 and the lowerdie 23 are moved in the molding step shown in FIG. 6. If the lower die23 is fixed and only the upper die 22 is moved as shown by the arrow 22a, however, pressure distribution inside the sintering material that ispressed by the upper die 22 is not the same as pressure distributioninside the sintering material that is placed on the lower die 23. Inother words, the pressure applied by the upper die 22 is transmitted tothe lower die 23 and is imparted to the inner die 20 and the outer die21. Therefore, a force applied from the upper die 22 to the molded body19 a in the molding step is not the same as a force that is applied fromthe lower die 23 to the molded body 19 a. Accordingly, there is adifference in density between the upper surface 19 d and the lowersurface 19 c of the molded body 19 a. Thus, there is a case where aportion of low density is contracted so that a warp is generated whenthe molded body 19 a having uneven density between the upper surface 19d and the lower surface 19 c is sintered. Therefore, the molding methodin which one of the upper die 22 and the lower die 23 is fixed and theother is moved cannot produce the sleeve 40 having a desired accuracy.In this second embodiment, the sleeve 40 having a desired accuracy canbe obtained because both the upper die 22 and the lower die 23 are movedfor molding.

With reference to FIG. 7 a molding die of the sleeve 1 in the firstembodiment will be described. Since the sleeve 1 has stepped endportions 1 b and 1 c, the lower die 33 of the molding die 52 includes anouter die 33 a, an inner die 33 b and a middle die 33 c that moveindependently. Other structures are the same as those shown in FIG. 6.

In the molding step, the molding die 52 is opened so that a gap betweenthe upper die 22 and the outer die 33 a as well as the inner die 33 b aswell as the middle die 33 c of the lower die 33 becomes a dimension 3L(not shown), and the space is filled with the sintering material. Next,the upper die 22 is moved in the direction of the arrow 22 a and theouter die 33 b, the inner die 33 a and the middle die 33 c are moved inthe direction of the arrow 33 e by the same distance so as to press thesintering material. After the sintering material is pressed to athickness of the dimension L, the inner die 33 b and the middle die 33 care further moved in the direction of the arrow 33 e so as to press amiddle portion 29 b of the molded body 29 a to be the dimension M. As aresult, the molded body 29 a is obtained. The middle portion 29 b of themolded body 29 a has a higher density than the density of the outercircumference portion 29 c. In addition, density is different betweenthe lower surface and the upper surface of the middle portion 29 b. Thereason is that when the middle portion 29 b is molded, the upper die 22is still while the inner die 33 b is moved in the direction of the arrow33 e so as to press the middle portion 29 b. When the molded body 29 ahaving uneven density among portions is sintered, a distortion will begenerated due to the unevenness of the density. Therefore, in order toobtain the sleeve 1 with a desired dimension of each portion from thesintered body obtained by sintering the molded body 29 a, a latheprocess is performed as shown in FIG. 8, for example. In this process,the sintered body 29 d is fixed to an air chuck 45 of a lathe machine sothat a turning tool 46 cuts the bearing bore 47 to a desired dimension.It is desirable to make the sintered body 29 d a little larger inexpectation of the cutting process. As understood from the moldingprocess described above, the sleeve 40 of the second embodiment canrealize high accuracy of dimensions after the sintering step. Therefore,the finishing process can be simple, and costs are reduced as a result.

In this case, we described hydrodynamic bearing having a shaft withflange. But it is possible to adapt this invention to a hydrodynamicbearing having a shaft without flange, as shown in FIGS. 11 and 12. Ahydrodynamic bearing shown in FIGS. 11 and 12 comprises a shaft 201, aninner sleeve 202, an outer sleeve 203, a thrust member 204, a base 210,a rotor hub 211, a rotor magnet 212, a motor stator 213 and discs 214. Athrust pressure generating groove 204A is formed on the thrust member204 of the hydrodynamic bearing having a shaft 201 without flange inFIG. 11. A thrust pressure generating groove 201A is formed on the endof the shaft 201 in FIG. 12.

While only selected embodiments have been chosen to illustrate thepresent invention, it will be apparent to those skilled in the art fromthis disclosure that various changes and modifications can be madeherein without departing from the scope of the invention as defined inthe appended claims. Furthermore, the foregoing descriptions of theembodiments according to the present invention are provided forillustration only, and not for the purpose of limiting the invention asdefined by the appended claims and their equivalents. Thus, the scope ofthe invention is not limited to the disclosed embodiments.

1. A hydrodynamic bearing device comprising: a sleeve having a bearingbore with a dynamic pressure generating groove formed on an innersurface of the bearing bore, the dynamic pressure generating groovebeing defined by at least one crest portion adjacent to the dynamicpressure generating groove; a shaft disposed in the bearing bore of thesleeve in a relatively rotatable manner; a thrust member opposed to anend of the shaft; a dynamic pressure generating groove formed on atleast one of opposing surfaces of the end of the shaft and the thrustmember; and working fluid filled in a gap between the shaft and thebearing bore, and between the end of the shaft and the thrust member,the sleeve being a sintered body including a sintering materialcontaining at least one selected from a group containing iron, an ironalloy, copper and a copper alloy, such that pores in the at least onecrest portion adjacent to the dynamic pressure generating groove of thesintered body are independent pores in which respective neighboringpores do not communicate with each other, and a diameter of theindependent pores is smaller than each of a width and a depth of the atleast one crest portion adjacent to the dynamic pressure generatinggroove.
 2. The hydrodynamic bearing device according to claim 1, whereinthe at least one of the opposing surfaces containing the dynamicpressure generating groove is a surface of the thrust member.
 3. Thehydrodynamic bearing device according to claim 1, wherein the thrustmember is made of a sintered metal.
 4. The hydrodynamic bearing deviceaccording to claim 1, wherein the at least one of the opposing surfacescontaining the dynamic pressure generating groove is a surface of thethrust member, and wherein the dynamic pressure generating groove is athrust dynamic pressure generating groove.
 5. A hydrodynamic bearingdevice comprising: a sleeve having a bearing bore with a dynamicpressure generating groove formed on an inner surface of the bearingbore, the dynamic pressure generating groove being defined by at leastone crest portion adjacent to the dynamic pressure generating groove; ashaft disposed in the bearing bore of the sleeve in a relativelyrotatable manner; a thrust member opposed to an end of the shaft; adynamic pressure generating groove formed on at least one of opposingsurfaces of the end of the shaft and the thrust member; and workingfluid filled in a gap between the shaft and the bearing bore, andbetween the end of the shaft and the thrust member the sleeve being asintered body including a sintering material containing at least oneselected from a group containing iron, an iron alloy, copper and acopper alloy, such that pores in the at least one crest portion adjacentto the dynamic pressure generating groove of the sintered body areindependent pores in which respective neighboring pores do notcommunicate with each other, and at least one of sintered body formingconditions including a molding pressure when the sintered body ismolded, sintering temperature and sintering period that are sinteringconditions, and an average grain size of metal grains of the sinteringmaterial being selected so that a diameter of the independent poresbecomes smaller than each of a width and a depth of the at least onecrest portion adjacent to the dynamic pressure generating groove.
 6. Thehydrodynamic bearing device according to claim 5, wherein the at leastone of the opposing surfaces containing the dynamic pressure generatinggroove is a surface of the thrust member.
 7. The hydrodynamic bearingdevice according to claim 5, wherein the thrust member is made of asintered metal.
 8. The hydrodynamic bearing device according to claim 5,wherein one of the sleeve and the thrust member includes an additionalopposing surface with an additional dynamic pressure generating groove,the additional dynamic pressure generating groove comprising a thrustdynamic pressure generating groove.